Wood Ultrastructure

Objective: Starting from a tree, we will zoom closer and closer in until we reach the molecular level.

Background information/helpful sites: For some good preliminary information, I suggest you browse the wood images used for the introductory wood couses at NC State. For some relatively cool ultrastructure pictures of macerated white pine and a piece of paper, click here.

Important concepts

1. Gymnosperms versus angiosperms. Gymnosperms, or conifers, have different cell types, and this is reflected in the general properties shown in the Table below. Gymnosperms typically have longer fibers relative to angiosperms.

2. Major cell types. For the most part, all we are concerned with are the cells within the woody portion of the tree (bark is composed of phloem cells but this portion of the tree is not of great economic value). Conifers are pretty much composed of tracheids and parenchyma cells. Tracheids are long and narrow (length:width = 50-100) whereas parenchyma are rectangular (length:width = 2-4). By volume, parenchyma cells make up 5-10% of the wood but only 1-3% of the mass.

Hardwoods have a much higher parenchyms cell content and is more variable; 10-35% by volume and a mass percentage of greater than 5%. Hardwood fibers include vessel elements and libriform fibers.

General Properties of North American Woods

Species	Fiber Length (mm)	Fiber Diameter (microns)	Wood Density (lbs/cubic foot)
Southern Region
Longleaf Pine	4.9	35-45	41
Shortleaf Pine	4.6	35-45	36
Loblolly Pine	3.6	35-45	36
Slash Pine	4.6	35-45	43
Northeast Region
Black Spruce	3.5	25-30	30
White Spruce	3.3	25-30	26
Jack Pine	3.5	28-40	30
Balsam Fir	3.5	30-40	25
Northwest Region
Douglas-fir	3.9	35-45	34
Western Hemlock	4.2	30-40	29
Redwood	6.1	50-65	25
Western Red Cedar	3.5	30-40	23
Hardwoods
Aspen	1.04	10-27	27
Birch	1.85	20-36	38
Beech	1.20	16-22	45
Oaks	1.40	14-22	46
Red Gum	1.70	20-40	34

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3. Wall structure and chemical components. The best way to discuss the wall structure is to first describe what the woody cell wall (or wood) is composed of. Remember, we tend to think of wood as an entity, but it is really composed of the cells mentioned above. The walls of these cells are what give wood and paper its properties and it is the chemicals/polymers that these walls are composed of that give the walls their properties.

 

Major Chemical Constituents of Trees

Cellulose­skeletal substance of wood and bark, a polymer of glucose.

Hemicelluloses and Pectins­matrix substances of wood and bark. These are polymers of several different carbohydrates.

Lignin­encrusting substance of wood and bark. A polymer composed of aromatic compounds (contains benzene rings).

Extractives­natural biocides found in wood and bark. A WIDE variety of compounds fall under this class of materials.

 

Layers in Woody Cell Wall

Middle Lamella---binds cells together, mostly lignin and pectin.

Primary Wall---polysaccharides and protein which have been covered by lignin. The fibers on the outside of the layer are random, but are more organized on the inside.

Secondary Walls---S1 (thin), S2 (thick) and S3 (thin). Mostly polysaccharides and lignin.

 

Stages of Cell Wall Formation in Wood

1. Cell division and formation of the cell plate.

2. Formation of a new primary cell wall.

3. Cell wall enlargement.

4. Cell wall thickening­secondary wall is formed.

5. Lignification of the middle lamella, primary and secondary walls.

6. Death of the cell.

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